Hi!
The fact that the voice of a boy changes more than the voice of a girl during the maturing process may be one reason for Littlefoot's frequent change of voice (6 times) if compared to Cera or Ducky. I'm not sure, but I think that the speaker of Petrie, Jeff Bennett is grown up, so his voice doesn't change anymore and he will be able to speak Petrie ever on. Spike doesn't speak very much, but I think his utterances are also made by a grownup.
As for longnecks, you are right that we didn't see much of the many longnecks Littlefoot's mother talked about in the original movie. Grownup longnecks can be seen in the background of some sequels (e.g. LBT 6), but in other sequels where one would expect to get a glimpse on the other longnecks of the Great Valley (especially LBT 4 and 10) don't show any other than Littlefoot and his grandparents.
